Austin, J. A., Jr., Schlager, W., et al., 1988 Proceedings of the Ocean Drilling Program, Scientific Results, Vol. 101 28. STRUCTURE AND EVOLUTION OF BAHAMIAN DEEP-WATER CHANNELS: INSIGHTS FROM IN-SITU GEOPHYSICAL AND GEOCHEMICAL MEASUREMENTS1 Colin F. Williams,2 Roger N. Anderson,2 and James A. Austin, Jr.3 ABSTRACT Two studies of well logs from Leg 101 of the Ocean Drilling Program provide new information concerning the un­ derlying structure of Bahamian deep-water channels. In the first study, subsidence calculations (based upon porosity and lithology logs from Site 626 in the Straits of Florida and oil tests located at Palm Beach, Key Largo, and Great Isaac Island) confirm the substantial (tens of kilometers) westward progradation of Great Bahama Bank, suggested by Austin, Schlager, et al. (1986). This progradation may have been going on since the late Oligocene. Subsidence calcula­ tions also show a consistent subsidence rate for the region in question, supporting the existence of a large Lower Creta­ ceous shallow-water carbonate platform underlying the northwestern Bahamas. In the second study, mineralogical in­ formation from an induced gamma spectroscopy log (GST) at Site 634 in Northeast Providence Channel was used to generate synthetic seismograms of the penetrated section. These seismograms accurately reproduce two reflectors visible on a nearby seismic line, tying these reflectors to specific depths and lithologies in the penetrated formation. Results of both studies illustrate that because physical properties of carbonate environments are facies-dependent, accurate geo­ physical models of these environments require detailed, site-specific measurements. INTRODUCTION ern Bahamas were the subject of a number of studies (e.g., Meyerhoff and Hatten, 1974; Tator and Hatfield, 1975; Mullins Leg 101 of the Ocean Drilling Program was the inauguration and Lynts, 1977; Schlager and Ginsburg, 1981; Sheridan et al., of the first permanent logging program in deep-sea drilling his­ 1981). Questions about the processes that created the Bahamian tory. Despite unstable hole conditions, through-pipe well log­ deep-water channels lie at the heart of the debate over two pop­ ging was completed successfully in Hole 626D (Straits of Flor­ ular hypotheses for the origin of the Bahamas—the graben and ida) and in Hole 634A (Northeast Providence Channel) (Fig. 1). the megabank hypotheses (Austin, Schlager, et al., 1986). The We report here the results of two studies that developed from graben hypothesis holds that horst and graben structures, cre­ these well-logging measurements. These studies not only pro­ ated by rifting that accompanied the initial formation of the At­ vide new information about the geologic structure and history lantic, underlie the modern Bahamian banks and channels, re­ of the Bahamas but also illustrate two methods by which geo­ spectively, and provide nearly complete tectonic morphological physical logs (when combined with data from drilling records control (Mullins and Lynts, 1977). The megabank hypothesis at­ and site surveys) can contribute insights into geological prob­ tributes today's arrangement of banks and channels to differen­ lems beyond simple characterizations of the physical properties tial growth and subsidence after a mid-Cretaceous(?) drowning of penetrated formations. of an extensive shallow-water carbonate platform province. This In the first study, in-situ porosity measurements from Hole province may have extended from the Bahamas across southern 626D and from three exploratory oil tests are the basis for exam­ Florida to northern Cuba and the Gulf of Mexico (see Schlager ining the post-Oligocene subsidence of the Straits of Florida and and Ginsburg, 1981). The differences between these two hypoth­ neighboring banks. This subsidence then is evaluated in terms of eses illustrate the division of Bahamas theorists into two groups, possible structures underlying the Straits of Florida and lateral with one group emphasizing the importance of erosional and migration of flanking carbonate banks. The second study de­ depositional processes and the other group relegating these pro­ scribes the development of a new synthetic seismogram technique cesses to a superficial role in affecting tectonically derived struc­ using geochemical well logs to generate a quantitative evaluation tures. of impedance changes with depth. This technique is applied to Leg 101 was drilled to supplement earlier studies and to ac­ logs from Hole 634A, providing correlations of prominent re­ quire new information about the formation and structure of the flectors on an adjacent multichannel seismic line with specific Bahamian deep-water channels. At the beginning of Leg 101, depths and ages in the hole. we hoped that two successful penetrations of reflectors (thought to represent the top of the drowned megabank) would provide a THE BAHAMAS CONTROVERSY clear, unambiguous solution to the problem of the Bahamian Distribution and orientation of Bahamian carbonate banks structural evolution. During events that have been documented and deep-water channels clearly are controlled by some combi­ elsewhere (Austin, Schlager, et al., 1986), drilling penetrated the nation of underlying basement structures and ancient sedimen­ top of a shallow-water carbonate platform only at Site 627, tary processes. The nature of these underlying structures and north of Little Bahama Bank (Fig. 1). This single penetration the degree to which they influenced the development of the mod- left open the question of whether the modern Bahamas lie atop a buried mid-Cretaceous megabank or are composed of a series of isolated carbonate banks, each with its own growth and sub­ mergence history. Although Eberli and Ginsburg (1987) recently Austin, J. A., Jr., Schlager, W., et al., Proc. ODP, Sci. Results, 101: College established that the post-Cretaceous structure of Great Bahama Station, TX (Ocean Drilling Program). 2 Bank is characterized by a set of banks whose margins have mi­ Borehole Research Group, Lamont-Doherty Geological Observatory, Pali­ grated laterally many kilometers, further correlation of Leg 101 sades, NY 10964. 3 Institute for Geophysics, University of Texas, 8701 Mopac Boulevard, Aus­ drilling results with data from other locations on the banks and tin, TX 78759-8345. in the channels is required to clarify the pre- and post-Creta- 439 C. F. WILLIAMS, R. N. ANDERSON, J. A. AUSTIN, JR. shows the corrected logs from Hole 626D. Corrections were ap­ plied for borehole size, eccentricity of the tool string, and atten­ uation caused by the presence of the drill pipe. These logs de­ r629*6632°7 Atlantic Ocean pict a number of oscillations in porosity, most likely from alter­ nating hard and soft layers in the formation. However, despite oPalm Beach these oscillations, a consistent decrease in porosity of approxi­ ida II Well £ mately 25% over the length of the hole occurs with depth. To develop geological models from Hole 626D logs, post­ 0^ cruise log analysis involved examining the subsidence of the 26°N Straits of Florida and relating that subsidence to structures un­ derlying and bounding the straits. If the straits formed as a re­ sult of erosional and depositional processes modifying a Creta­ pKey Largo ceous megabank, then predictable observations should follow Well after calculating the subsidence rates at Sites 626 and the Great Isaac, Palm Beach, and Key Largo wells. First, when corrected for varying post-Cretaceous sedimentation rates, subsidence rates on either side of the Straits of Florida should be about the 24° - L 632 100 km same. Second, subsidence at Site 626 should be slightly greater iw /» v than that predicted by sediment loading and compaction alone. 80°W 76° This follows from the supposition that the underlying mega­ Long Island o bank is subsiding at a regionally uniform rate. This rate should Well be controlled by larger sediment loads on the banks, causing the Figure 1. Location map of Leg 101 sites and nearby exploratory wells in deep-water channels to show a steadily increasing deviation from the Bahamas. the local isostatic equilibrium predicted by deep-water sediment loading. If the straits are underlain by a grabenlike structure, motion along the faults that formed the straits may have ceased ceous history of the Bahamas. Here, we focus on approaches more than 100 m.y. ago, and the presence of these faults might for establishing the validity of seismic ties among various deep not show up during recent subsidence history. However, if these penetration sites in the northwestern Bahamas, thereby con­ faults were active during the time covered by the subsidence straining their structure and evolution. data, then the subsidence record at Site 626 should clearly indi­ cate fault motion. LOG-DERIVED SUBSIDENCE AND STRUCTURE Figure 4 shows the uncorrected sedimentation-rate/subsidence OF THE STRAITS OF FLORIDA curves for Site 626 and the Great Isaac, Key Largo, and Palm Site 626, located in the eastern half of the Straits of Florida Beach wells. Palm Beach and Key Largo display almost identical between Miami and the Bimini Island group (Fig. 1), was drilled subsidence patterns, while the subsidence at Site 626 is about with the intention (1) of penetrating the top of the drowned 150 m greater since the beginning of the late Oligocene. Over mid-Cretaceous(?) shallow-water carbonate platform and (2) of the same interval, the subsidence at Great Isaac is greater by documenting the history of local Gulf Stream flow (Austin, more than a factor of 3 than that at the other sites. Thus, it ap­ Schlager, et al., 1986). Although drilling operations had to be pears that subsidence on the eastern side of the straits increased suspended before reaching our desired goals, the proximity of more than that on the western side. However, this increased sub­ oil test wells on either side of the straits and seismic-jump corre­ sidence at Great Isaac may simply have been caused by greater lations of those wells with Site 626 allowed us to study the struc­ sedimentation rates.